Metal bump joint structure and methods of forming

ABSTRACT

A structure comprises a first semiconductor chip with a first metal bump and a second semiconductor chip with a second metal bump. The structure further comprises a solder joint structure electrically connecting the first semiconductor chip and the second semiconductor chip, wherein the solder joint structure comprises an intermetallic compound region between the first metal bump and the second metal bump, wherein the intermetallic compound region is with a first height dimension and a surrounding portion formed along exterior walls of the first metal bump and the second metal bump, wherein the surrounding portion is with a second height dimension, and wherein the second height dimension is greater than the first height dimension.

This application is a divisional of U.S. patent application Ser. No.13/656,968, filed on Oct. 22, 2012, entitled “Metal Bump JointStructure,” which application is hereby incorporated herein by referencein its entirety.

BACKGROUND

The semiconductor industry has experienced rapid growth due tocontinuous improvements in the integration density of a variety ofelectronic components (e.g., transistors, diodes, resistors, capacitors,etc.). For the most part, this improvement in integration density hascome from repeated reductions in minimum feature size, which allows morecomponents to be integrated into a given area. As the demand for evensmaller electronic devices has grown recently, there has grown a needfor smaller and more creative packaging techniques of semiconductordies.

As semiconductor technologies further advance, chip-scale or chip-sizepackaging based semiconductor devices have emerged as an effectivealternative to further reduce the physical size of a semiconductor chip.In a chip-scale packaging based semiconductor device, the packaging isgenerated on the die with contacts provided by a variety of bumpsincluding copper bumps, solder balls and/or the like. Much higherdensity can be achieved by employing chip-scale packaging basedsemiconductor devices. Furthermore, chip-scale packaging basedsemiconductor devices can achieve smaller form factors,cost-effectiveness, increased performance, lower power consumption andlower heat generation.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a cross sectional view of a semiconductor device inaccordance with various embodiments of the present disclosure; and

FIG. 2 illustrates in detail a cross sectional view of the semiconductorchips shown in FIG. 1 in accordance with various embodiments of thepresent disclosure.

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures aredrawn to clearly illustrate the relevant aspects of the variousembodiments and are not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently embodiments are discussed indetail below. It should be appreciated, however, that the presentdisclosure provides many applicable inventive concepts that can beembodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the disclosure, and do not limit the scope of the disclosure.

The present disclosure will be described with respect to embodiments ina specific context, a metal bump joint structure at an interface betweentwo semiconductor dies. The disclosure may also be applied, however, toa variety of joint structures of semiconductor devices. Hereinafter,various embodiments will be explained in detail with reference to theaccompanying drawings.

FIG. 1 illustrates a cross sectional view of a semiconductor device inaccordance with various embodiments of the present disclosure. Thesemiconductor device 100 includes a first semiconductor component 130and a second semiconductor component 150. As shown in FIG. 1, the secondsemiconductor component 150 is stacked on top of the first semiconductorcomponent 130 through a metal bump joint structure. The metal bump jointstructure includes a first metal bump 132 formed over the firstsemiconductor component 130, a second metal bump 152 formed over thesecond semiconductor component 150 and a solder joint structure 144 atthe interface between two semiconductor components 130 and 150. Itshould be noted while FIG. 1 illustrates one metal bump (e.g., metalbump 132) at each semiconductor component (e.g., first semiconductorcomponent 130), both semiconductor components 130 and 150 canaccommodate any number of metal bumps.

In some embodiments, both semiconductor components 130 and 150 are asemiconductor chip. In alternative embodiments, the semiconductorcomponents 130 and 150 may be a package substrate, an interposer and/orthe like. For simplicity, throughout the description, the firstsemiconductor component 130 and the second semiconductor component 150are alternatively referred to as a first semiconductor chip 130 and asecond semiconductor chip 150 respectively. The detailed structures ofthe first semiconductor chip 130 and the second semiconductor chip 150will be described below with respect to FIG. 2.

The metal bumps 132 and 152 may be formed of copper. FIG. 1 shows bothfirst metal bump 132 and the second metal bump 152 are of a rectangularshape. It should be noted that the shapes of the metal bumps 132 and 152shown in FIG. 1 are merely an example. A person skilled in the art willrecognize that the disclosure is applicable to a variety ofsemiconductor bumps such as ladder shaped bumps, which are commonlyknown as ladder connectors.

As shown in FIG. 1, barrier layers 134 and 154 are formed over the metalbumps 132 and 152 respectively. The barrier layers 134 and 154 may beformed of nickel (Ni), gold (Au), silver (Ag), palladium (Pd), Platinum(Pt), nickel-palladium-gold, nickel-gold, any combinations thereofand/or the like. The barrier layer 134 and 154 may be formed usingsuitable fabrication techniques such as plating and/or the like.

Prior to a reflow process through which the first semiconductor chip 130and the second semiconductor chip 150 are bonded together, a solder ball(not shown) may be formed on either the first semiconductor chip 130 orthe second semiconductor chip 150. Alternatively, the solder ball may beformed on one chip and a thin solder layer (not shown) may be formed onthe other chip.

The solder ball may be made of any of suitable materials. In accordancewith some embodiments, the solder ball may comprise SAC405. SAC405comprises 95.5% Sn, 4.0% Ag and 0.5% Cu.

The first semiconductor chip 130 and the second semiconductor chip 150may be bonded together through any suitable flip-chip bondingtechniques. Solder ball thus joins the first semiconductor chip 130 andthe second semiconductor chip 150 together. A reflow process isperformed to melt solder ball to form the solder joint structure 144shown in FIG. 1. In some embodiments, the reflow process may beperformed at a temperature in a range from about 220 degrees C. to about280 degrees C.

After the reflow process, due to a metallurgical reaction between solder(e.g., solder ball) and nickel (e.g., barrier layers 134 and 154 formedof nickel), an intermetallic compound (IMC) layer 142 is formed at theinterface between the first metal bump 132 and the second metal bump152. In some embodiments, the IMC layer 142 may contain Ni₃Sn₄. As shownin FIG. 1, the solder joint structure 144 may include two portions,namely the IMC portion 142 and a surrounding portion 146. As indicatedby a dashed rectangle shown in FIG. 1, the IMC portion 142 is mainlylocated between the first barrier layer 134 and the second barrier layer154. The surrounding portion 146 surrounds the exterior walls of thefirst metal bump 132 and the second metal bump 152.

FIG. 1 illustrates a cross sectional view of the surrounding portion146. In a top view (not shown) of the semiconductor device 100, thesurrounding portion 146 is like a belt wrapping around the interfacebetween the first metal bump 132 and the second metal bump 152. Thesurrounding portion 146 may be of a uniform thickness at each crosssection. However, due to process and operation variations, thethicknesses at various cross sections of the surrounding portion 146 maybe not uniform. Assume that the cross sectional view of FIG. 1 is takenat the thinnest portion of the surrounding portion 146.

As shown in FIG. 1, the height of the IMC layer 142 is defined as H1.The horizontal distance between the first barrier layer 134 and theexterior surface of the solder joint structure 144 is defined as D1.Likewise, the horizontal distance between the second barrier layer 154and the exterior surface of the solder joint structure 144 is defined asD2.

From the cross sectional view shown in FIG. 1, the surrounding portion146 is of a half ellipse shape. The height of the half ellipse shape isdefined as D3. Throughout the description, the height of the halfellipse shape is alternatively referred to as the height of thesurrounding portion 146.

In order to have a reliable solder joint, the dimensions of the solderjoint structure 144 may be subject to the following restrictions. Afirst ratio of D1 to H1 is greater than 1. Likewise, a second ratio ofD2 to H1 is greater than 1 and a third ratio of D3 to H1 is greater than1.

In some embodiments, H1 is a range from about 3 um to about 5 um. D1, D2and D3 are in a range from about 4 um to about 6 um.

One advantageous feature of having the restrictions shown above is thatthe dimension restrictions help to prevent cracks from propagating alongthe interface between the first metal bump 132 and the second metal bump152. Furthermore, it was observed in the reliability tests (e.g.,thermal cycles performed on the stacked semiconductor chips) that withthe solder joint structure 144 shown in FIG. 1, the reliability of theresulting stacked semiconductor chips is significantly improved.

FIG. 2 illustrates in detail a cross sectional view of the semiconductorchips shown in FIG. 1 in accordance with various embodiments of thepresent disclosure. As shown in FIG. 1, the semiconductor device 100includes the first semiconductor chip 130 and the second semiconductorchip 150, wherein the first semiconductor chip 130 is coupled to thesecond semiconductor chip 150 through a solder joint. The firstsemiconductor chip 130 and the second semiconductor chip 150 may be of asame structure. For simplicity, only the detailed structure of the firstsemiconductor chip 130 is illustrated in FIG. 2.

As shown in FIG. 2, the first semiconductor chip 130 may comprise asubstrate 102 and a plurality of interconnect components formed over thesubstrate 102. The substrate 102 may be formed of silicon, although itmay also be formed of other group III, group IV, and/or group Velements, such as silicon, germanium, gallium, arsenic, and combinationsthereof and/or the like.

The substrate 102 may also be in the form of silicon-on-insulator (SOI).The SOI substrate may comprise a layer of a semiconductor material(e.g., silicon, germanium and/or the like) formed over an insulatorlayer (e.g., buried oxide or the like), which is formed in a siliconsubstrate. In addition, other substrates that may be used includemulti-layered substrates, gradient substrates, hybrid orientationsubstrates and/or the like. The substrate 102 may further comprise avariety of electrical circuits (not shown). The electrical circuitsformed on the substrate 102 may be any type of circuitry suitable for aparticular application.

In accordance with some embodiments, the electrical circuits may includevarious n-type metal-oxide semiconductor (NMOS) and/or p-typemetal-oxide semiconductor (PMOS) devices such as transistors,capacitors, resistors, diodes, photo-diodes, fuses and the like. Theelectrical circuits may be interconnected to perform one or morefunctions. The functions may include memory structures, processingstructures, sensors, amplifiers, power distribution, input/outputcircuitry or the like. One of ordinary skill in the art will appreciatethat the above examples are provided for illustrative purposes only tofurther explain applications of the present disclosure and are not meantto limit the present disclosure in any manner.

An interlayer dielectric layer 104 is formed on top of the substrate102. The interlayer dielectric layer 104 may be formed, for example, ofa low-K dielectric material, such as silicon oxide. The interlayerdielectric layer 104 may be formed by any suitable method known in theart, such as spinning, chemical vapor deposition (CVD) and plasmaenhanced chemical vapor deposition (PECVD) and/or the like. It shouldalso be noted that one skilled in the art will recognize that theinterlayer dielectric layer 104 may further comprise a plurality ofdielectric layers.

A bottom metallization layer 106 and a top metallization layer 108 areformed over the interlayer dielectric layer 104. As shown in FIG. 2, thebottom metallization layer 106 comprises a first metal line 126.Likewise, the top metallization layer 108 comprises a second metal line128. Metal lines 126 and 128 are formed of metal materials such ascopper or copper alloys and the like. The metallization layers 106 and108 may be formed through any suitable techniques (e.g., deposition,damascene and the like). Generally, the one or more inter-metaldielectric layers and the associated metallization layers are used tointerconnect the electrical circuits in the substrate 102 to each otherto form functional circuitry and to further provide an externalelectrical connection.

It should be noted while FIG. 2 shows the bottom metallization layer 106and the top metallization layer 108, one skilled in the art willrecognize that one or more inter-metal dielectric layers (not shown) andthe associated metallization layers (not shown) are formed between thebottom metallization layer 106 and the top metallization layer 108. Inparticular, the layers between the bottom metallization layer 106 andthe top metallization layer 108 may be formed by alternating layers ofdielectric (e.g., extreme low-k dielectric material) and conductivematerials (e.g., copper).

A dielectric layer 110 is formed on top of the top metallization layer108. As shown in FIG. 2, a top metal connector 124 is embedded in thedielectric layer 110. In particular, the top metal connector provides aconductive channel between the metal line 128 and the electricalconnection structure of the semiconductor device. The top metalconnector 124 may be made of metallic materials such as copper, copperalloys, aluminum, silver, gold and any combinations thereof. The topmetal connector 124 may be formed by suitable techniques such as CVD.Alternatively, the top metal connector 124 may be formed by sputtering,electroplating and/or the like.

A first passivation layer 112 is formed on top of the dielectric layer110. In accordance with an embodiment, the first passivation layer 112is formed of non-organic materials such as un-doped silicate glass,silicon nitride, silicon oxide and the like. Alternatively, the firstpassivation layer 112 may be formed of low-k dielectric such as carbondoped oxide and the like. In addition, extreme low-k (ELK) dielectricssuch as porous carbon doped silicon dioxide can be employed to form thefirst passivation layer 112. The first passivation layer 112 may beformed through any suitable techniques such as CVD and/or the like. Asshown in FIG. 2, there may be an opening formed in the first passivationlayer 112. The opening is used to accommodate the bond pad 116, whichwill be discussed in detail below.

A second passivation layer 114 is formed on top of the first passivationlayer 112. The second passivation layer 114 may be similar to the firstpassivation layer 112, and hence is not discussed in further detail toavoid unnecessary repetition. As shown in FIG. 2, a bond pad 116 isformed in the openings of the first passivation and second passivationlayers. In accordance with some embodiments, the bond pad 116 may beformed of aluminum. For simplicity, throughout the description, the bondpad 116 may be alternatively referred to as an aluminum pad 116.

The aluminum pad 116 may be enclosed by the first and second passivationlayers 112 and 114. In particular, a bottom portion of the aluminum pad116 is embedded in the first passivation layer 112 and a top portion ofthe aluminum pad 116 is embedded in the second passivation layer 114.The first and second passivation layers 112 and 114 overlap and seal theedges of the aluminum pad 116 so as to improve electrical stability bypreventing the edges of the aluminum pad 116 from corrosion. Inaddition, the passivation layers 112 and 114 may help to reduce theleakage current of the semiconductor device.

A polymer layer 118 is formed on top of the second passivation layer114. The polymer layer 118 is made of polymer materials such as epoxy,polyimide and the like. In particular, the polymer layer 118 maycomprise photo-definable polyimide materials such as HD4104. Forsimplicity, throughout the description, the polymer layer 118 may bealternatively referred to as the PI layer 118. The PI layer 118 may bemade by any suitable method known in the art such as spin coating and/orthe like. A redistribution layer (not shown) may be formed in thesemiconductor device 100 if the bond pads are relocated to newlocations. The redistribution layer provides a conductive path betweenthe metal lines (e.g., metal line 128) and the redistributed bond pads.The operation principles of redistribution layers are well known in theart, and hence are not discussed in detail herein.

The PI layer 118 is patterned to form a plurality of openings.Furthermore, various under bump metal (UBM) structures (not shown) areformed on top of the openings. The UBM structures are employed toconnect the aluminum pads (e.g., aluminum pad 116) with various inputand output terminals (e.g., metal bump 132). The UBM structures may beformed by any suitable techniques such as electroplating. Otherprocesses of formation such as sputtering, evaporation, PECVD and/or thelike may alternatively be used depending upon the desired materials.

In some embodiments, the metal bump 132 may be a copper bump. The copperbump may be of a height of approximately 16 um. A variety ofsemiconductor packaging technologies such as sputtering, electroplatingand photolithography can be employed to form the metal bump 132. Asknown in the art, in order to insure the reliable adhesion andelectrical continuity between the copper bump and the bond pad 116,additional layers including a barrier layer, an adhesion layer and aseed layer (not shown respectively) may be formed between the metal bump132 and the bond pad 116.

Although embodiments of the present disclosure and its advantages havebeen described in detail, it should be understood that various changes,substitutions and alterations can be made herein without departing fromthe spirit and scope of the disclosure as defined by the appendedclaims.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the present disclosure, processes, machines,manufacture, compositions of matter, means, methods, or steps, presentlyexisting or later to be developed, that perform substantially the samefunction or achieve substantially the same result as the correspondingembodiments described herein may be utilized according to the presentdisclosure. Accordingly, the appended claims are intended to includewithin their scope such processes, machines, manufacture, compositionsof matter, means, methods, or steps.

What is claimed is:
 1. A method comprising: forming a first metal bumpon a surface of a first semiconductor component; depositing a firstbarrier layer over the first metal bump, a material of the first barrierlayer being different from a material of the first metal bump; forming asecond metal bump on a surface of a second semiconductor component;depositing a second barrier layer over the second metal bump, a materialof the second barrier layer being different from a material of thesecond metal bump; and bonding the second semiconductor component to thefirst semiconductor component using a reflow process, wherein solder ismelted to form a solder joint structure comprising: an intermetalliccompound region formed between the first barrier layer and the secondbarrier layer, the intermetallic compound region comprising at least oneelement of the material of the first barrier layer and/or of thematerial of the second barrier layer and comprising at least one elementof the solder, the intermetallic compound region having a firstdimension in a first direction from the first barrier layer to thesecond barrier layer; and a surrounding portion comprising the solderformed along exterior walls of the first metal bump and the second metalbump, the surrounding portion having a second dimension in a seconddirection from a point aligned with at least one of respective walls ofthe first barrier layer and the second barrier layer to a nearestexterior surface of the surrounding portion, the first direction beingperpendicular to the second direction, the second dimension beinggreater than the first dimension.
 2. The method of claim 1, furthercomprising: forming the solder on the first barrier layer; and applyingthe reflow process to melt the solder to form the solder jointstructure.
 3. The method of claim 1, further comprising: forming thesolder on the second barrier layer; and applying the reflow process tomelt the solder to form the solder joint structure.
 4. The method ofclaim 1, further comprising: forming a first metal layer over asubstrate; forming a first dielectric layer on the first metal layer;and forming a second metal layer on the first dielectric layer.
 5. Themethod of claim 4, further comprising: forming a first passivation layerformed over the second metal layer; forming a second passivation layerover the first passivation layer; forming a bond pad, wherein the bondpad is embedded in the first passivation layer and the secondpassivation layer; and forming a polymer layer on the second passivationlayer.
 6. The method of claim 1, wherein the material of the firstbarrier layer and the material of the second barrier layer is nickel. 7.The method of claim 1, wherein the intermetallic compound regioncomprises Ni₃Sn₄.
 8. The method of claim 1, wherein the surroundingportion is a half ellipse shape and comprises: a first distance from thewall of the first barrier layer to a surface of the half ellipse, thefirst distance being in a direction parallel to the second direction,and a second distance from the wall of the second barrier layer to thesurface of the half ellipse shape, the second distance being in adirection parallel to the second direction, each of the first distanceand the second distance being greater than the first dimension.
 9. Amethod comprising: forming a first metal bump on a side of a firstsubstrate; forming a first barrier layer on the first metal bump, thefirst barrier layer being a material distinct from a material of thefirst metal bump; after forming the first barrier layer, forming asolder on the first barrier layer; and bonding the first metal bump to asecond metal bump on a second substrate by reflowing the solder, asecond barrier layer being on the second metal bump, the second barrierlayer being a material distinct from a material of the second metalbump, a surrounding portion of the solder being formed along exteriorwalls of the first metal bump and the second metal bump, the surroundingportion of the solder having a lateral dimension from a point alignedwith at least one of respective walls of the first barrier layer and thesecond barrier layer to a nearest exterior surface of the surroundingportion of the solder, the lateral dimension being in a directionparallel to the side of the first substrate, a vertical dimension beingfrom the first barrier layer to the second barrier layer, the verticaldimension being in a direction perpendicular to the side of the firstsubstrate, the lateral dimension being greater than the verticaldimension.
 10. The method of claim 9, wherein the reflowing forms anintermetallic compound region between the first barrier layer and thesecond barrier layer, the intermetallic compound region comprising acompound, the compound comprising at least one element of the materialof the first barrier layer and/or the material of the second barrierlayer and comprising an element of the solder.
 11. The method of claim10, wherein the compound comprises Ni₃Sn₄.
 12. The method of claim 9,wherein the material of the first barrier layer and the material of thesecond barrier layer is nickel.
 13. The method of claim 9 furthercomprising: forming the second metal bump on the second substrate; andforming the second barrier layer on the second metal bump.
 14. Themethod of claim 9, wherein the surrounding portion is a half ellipseshape and comprises: a first distance in a lateral direction from thewall of the first barrier layer to a surface of the half ellipse, and asecond distance in a lateral direction from the wall of the secondbarrier layer to the surface of the half ellipse shape, each of thefirst distance and the second distance being greater than the verticaldimension.
 15. A method comprising: forming a first barrier layer on afirst metal bump on a side of a first substrate, the first barrier layercomprising a first material, the first metal bump comprising a secondmaterial, the first material being different from the second material;forming a second barrier layer on a second metal bump on a secondsubstrate, the second barrier layer comprising a third material, thesecond metal bump comprising a fourth material, the third material beingdifferent from the fourth material; and after forming the first barrierlayer and the second barrier layer, reflowing solder between the firstbarrier layer and the second barrier layer, the reflowing reacting thesolder with at least one of the first material and the third material toform an intermetallic compound between the first barrier layer and thesecond barrier layer, an unreacted portion of solder surrounding theintermetallic compound and exterior walls of the first metal bump andthe second metal bump, a lateral dimension being from a point alignedwith at least one of the exterior walls of the first barrier layer andthe second barrier layer to a nearest exterior surface of the unreactedportion, the lateral dimension being in a direction parallel to the sideof the first substrate, a vertical dimension being from the firstbarrier layer to the second barrier layer, the vertical dimension beingin a direction perpendicular to the side of the first substrate, thelateral dimension being greater than the vertical dimension.
 16. Themethod of claim 15, wherein the intermetallic compound is Ni₃Sn₄. 17.The method of claim 15, wherein the first material and the thirdmaterial is nickel.
 18. The method of claim 15 further comprising:forming the first metal bump on the first substrate; and forming thesecond metal bump on the second substrate.
 19. The method of claim 15,wherein the unreacted portion is a half ellipse shape and comprises: afirst distance in a lateral direction from the wall of the first barrierlayer to a surface of the half ellipse, and a second distance in alateral direction from the wall of the second barrier layer to thesurface of the half ellipse shape, each of the first distance and thesecond distance being greater than the vertical dimension.
 20. Themethod of claim 15, wherein the reflowing is performed at a temperaturein a range from 220° C. to 280° C.
 21. A method comprising: depositing afirst barrier layer on a first metal bump, the first metal bump being ona first semiconductor component, the first metal bump comprising a firstmaterial composition, the first barrier layer comprising a secondmaterial composition different from the first material composition;depositing a second barrier layer on a second metal bump, the secondmetal bump being on a second semiconductor component, the second metalbump comprising a third material composition, the second barrier layercomprising a fourth material composition different from the thirdmaterial composition; and reflowing solder between the first barrierlayer and the second barrier layer, the reflowing reacting at least oneelement in the second material composition and/or the fourth materialcomposition with at least one element in the solder to form anintermetallic compound in an intermediate region between the firstbarrier layer and the second barrier layer, the intermediate regionhaving a first dimension from the first barrier layer to the secondbarrier layer and in a first direction, a surrounding portion comprisingthe solder being formed along exterior walls of the first metal bump andthe second metal bump, the surrounding portion having a second dimensionfrom a point aligned with at least one of respective walls of the firstbarrier layer and the second barrier layer to a nearest exterior surfaceof the surrounding portion and in a second direction, the firstdirection being perpendicular to the second direction, the seconddimension being greater than the first dimension.
 22. The method ofclaim 21, further comprising forming the solder on the first barrierlayer before the reflowing.
 23. The method of claim 21, furthercomprising forming the solder on the first barrier layer and the secondbarrier layer before the reflowing.
 24. The method of claim 21, furthercomprising: forming a first metal layer over a substrate, the firstsemiconductor component comprising the substrate; forming a firstdielectric layer on the first metal layer; and forming a second metallayer on the first dielectric layer.
 25. The method of claim 24, furthercomprising: forming a first passivation layer formed over the secondmetal layer; forming a second passivation layer over the firstpassivation layer; forming a bond pad, wherein the bond pad is embeddedin the first passivation layer and the second passivation layer, thefirst metal bump being formed on the bond pad; and forming a polymerlayer on the second passivation layer.
 26. The method of claim 21,wherein the material of the first barrier layer and the material of thesecond barrier layer is nickel.
 27. The method of claim 21, wherein theintermetallic compound comprises Ni₃Sn₄.
 28. The method of claim 21,wherein the surrounding portion is a half ellipse shape and comprises: afirst distance from the wall of the first barrier layer to a surface ofthe half ellipse, the first distance being in a direction parallel tothe second direction, and a second distance from the wall of the secondbarrier layer to the surface of the half ellipse shape, the seconddistance being in a direction parallel to the second direction, each ofthe first distance and the second distance being greater than the firstdimension.